This application claims priority to Japanese Patent Application No. 2001-259155 filed on Aug. 29, 2001.
1. Field of the Invention
The present invention relates to methods and instruments for measuring the magnetic and electric characteristics of devices that vary with time such as measuring a magnetic field and magnetizing current (excitation current) in a magnetic write head operating at a high frequency.
2. Description of the Background
An analytical method using electron beam concerning material properties is conventionally used for the analysis and measurement of various characteristics of magnetic materials because it has a high spatial resolution (i.e., detect small displacements) and an electron beam is sensitive to magnetism. However, the speed of operation of magnetic devices, such as magnetic heads, has been increasing recently. To analyze the properties of these high speed devices, not only a high spatial resolution, but also the ability to measure magnetic characteristics that vary at high speed (i.e., xe2x80x9chigh time resolutionxe2x80x9d) is required.
A method for measuring timewise varying magnetic characteristics using an electron beam is known and described in, for example, IEEE Transaction of Magnetics, Vol. 28, No. 2, March 1992. According to this conventional method, a stroboscopic electron beam is generated and is passed through a magnetic field synchronized with the period of the alternating current of a write head. Only information at a specific phase in a magnetic field distribution, which varies periodically, is extracted. The phase can be arbitrarily determined, and the magnetic field distribution can be measured at any time in the period of the alternating current.
To monitor a magnetizing current (excitation current) of a magnetic head operating at a high frequency, there is widely adopted a method of measuring the waveform of an electric current flowing through a wire using a current probe. According to this method, a generated electromotive force (emf) is detected by a current transformer installed in the vicinity of the wire, with a terminated end connected to, for example, an oscilloscope for display as a waveform.
Another method used to measure a timewise varying magnetizing current in a magnetic head is described in Japanese Published Unexamined Patent Application No. 4-372887. According to this method, as shown in FIG. 10, a resistor 4 is inserted in series with a magnetic head 28, and a voltage developed across the resistor is fed to a deflection electrode 5. A stroboscopic electron beam pulse 11 having the same timing as that for magnetic field measurement is passed through the deflection electrode 5 and a displacement of the beam deflection is detected by a position sensor 15 and is converted to a current value. That is, an electric current is measured in terms of an electric field. In this method, which does not use a current probe, the electron beam 10 is made into a stroboscopic electron beam 11, and the stroboscopic electron beam is irradiated in conformity with the period of the alternating current of the magnetic head 28 which varies repeatedly. Only information at a specific phase is extracted, and sampling is made while gradually changing the timing thereof. Using this method, it has been possible to measure the waveform of a magnetizing current over a specific time range.
In the above conventional method using an electron beam, the electron beam 10 must be made into a stroboscopic electron beam 11 by blanking with use of a high-speed deflector 24 and an aperture 25, and the pulse width of the stroboscopic electron beam 11 is determined in terms of a time resolution. For example, given that one operation period is T and further given that a time-resolution necessary for the measurement thereof is one-twentieth of one period, a pulse width of no more than T/20 or less is required.
A high-speed deflection electrode 24 and an aperture plate 25 having a small aperture are in many cases employed for generating an electron beam pulse. Herein, it is assumed that the electron beam converged on the aperture is deflected at xc2x11V. When the ratio between the displacement of the beam deflection and the diameter of the aperture is 5:1, and when a pulse width of 1 ns is needed, a xe2x80x9creversalxe2x80x9d time (rise time or fall time) of the voltage signal of approximately 5 ns is necessary. If a pulse width of 0.1 ns is needed, a reversal time of 0.5 ns is necessary.
With the recent increase in the driving frequency of magnetic devices such as magnetic heads, an increased measurement time resolution has come to be demanded, and the pulse width of the required electron beam is being continually reduced. However, when the pulse width is decreased using the above method, if a signal with a short reversal time is input to the deflection electrode, a reflection or overshoot caused by the wire or load may occur, and the signal may therefore become a nonlinear input signal. As a result, it is not possible to deflect the electron beam with an accurate reversal time. Thus, this conventional method is limited in that an electron beam pulse cannot be accurately generated with a sufficiently short pulse width.
The present invention preferably provides instruments and methods to effect the measurement of a high-frequency magnetic field with a high time-resolution without using a quickly reversing voltage signal, that is, without converting the electron beam into a stroboscopic pulse.
Conventional methods have been limited in that if a current probe is used to monitor a high-frequency current used to operate a magnetic head, the load thereof exerts an influence on the current circuit itself, causing the detected waveform to be distorted. This limitation makes it impossible to accurately measure a current waveform in a magnetized state. The present invention, therefore, also preferably provides devices and methods to realize a highly sensitive and accurate current monitor without using a current transformer and, hence, without burdening a current circuit.
According to one aspect of the present invention, to address the above-mentioned limitations of the conventional methods, there is preferably provided an instrument for measuring a magnetic field comprising: a source of a charged particle beam; means for converging the charged particle beam; means for generating a magnetic field to be measured; means for directing the charged particle beam through a desired position in the magnetic field to be measured; means for deflecting the charged particle beam in a direction different from a beam deflection direction caused by the magnetic field to be measured; means for providing a control signal to the means for deflecting the charged particle beam in the different direction; means for enlarging the displacement of deflection of the deflected charged particle beam; a two-dimensional sensor for detecting the enlarged displacement of beam deflection; and a display for displaying the track of the charged particle beam detected by the sensor. This embodiment may also be employed with just the charged particle beam, the magnetic head, the deflection means for deflecting the beam in a different angle from the magnetic head, and same type of sensor and/or display.
According to another aspect of the present invention, the above instrument may further comprise means for switching (xe2x80x9conxe2x80x9d and xe2x80x9coffxe2x80x9d) the irradiation of the charged particle beam into the magnetic field to be measured at a position between the source of the charged particle beam and the means for generating the magnetic field to be measured and means that control the irradiation of the charged particle beam into the magnetic field, and non-irradiation thereof, by providing a signal to the means for switching the irradiation.
According to a further aspect of the present invention there is provided a method for measuring a magnetic field by utilizing an interaction with a charged particle beam comprising the steps of: generating a charged particle beam from a source; converging the charged particle beam; generating a magnetic field to be measured using of a signal; allowing the converged charged particle beam to pass through a predetermined position in the magnetic field; deflecting the charged particle beam in a direction different from a direction of deflection of the charged particle beam caused by the magnetic field while controlling the deflection with the signal fed; causing the magnetic field to be measured and the signal for deflecting the charged particle beam to change with a predetermined period; enlarging the displacement of deflection of the deflected charged particle beam; detecting the enlarged displacement of beam deflection with use of a two-dimensional sensor; and displaying a detected track of the charged particle beam. The steps in the method may also be compacted as was described above with respect to the instrument.